Removal of Mercury(II) from Aqueous Solutions by Adsorption on Poly(1-amino-5-chloroanthraquinone) Nanofibrils: Equilibrium, Kinetics, and Mechanism Studies

1Research Center for Analysis and Measurement, Kunming University of Science and Technology, Kunming 650093, China2School of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China3State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China4Faculty of Science, Kunming University of Science and Technology, Kunming 650093, China

Abstract

Poly(1-amino-5-chloroanthraquinone) (PACA) nanofibrils were applied as novel nanoadsorbents for highly toxic mercury removal from aqueous solutions. A series of batch adsorption experiments were conducted to study the effect of adsorbent dose, pH, contact time, and metal concentration on Hg(II) uptake by PACA nanofibrils. Kinetic data indicated that the adsorption process of PACA nanofibrils for Hg(II) achieved equilibrium within 2 h following a pseudo-second-order rate equation. The adsorption mechanism of PACA nanofibrils for Hg(II) was investigated by Fourier transform-infrared (FT-IR) spectra and X-ray photoelectron spectroscopy (XPS) analyses. The adsorption isotherm of Hg(II) fitted well the Langmuir model, exhibiting superb adsorption capacity of 3.846 mmol of metal per gram of adsorbent. Lastly, we found out that the as-synthesized PACA nanofibrils are efficient in Hg(II) removal from real wastewater. Furthermore, five consecutive adsorption-desorption cycles demonstrated that the PACA nanofibrils were suitable for repeated use without considerable changes in the adsorption capacity.

1. Introduction

Heavy metal ions are highly toxic even at low concentrations and can accumulate in living organisms, causing several disorders and diseases. Unlike organic pollutants, heavy metals are nondegradable. Once in the food chain, they are able to accumulate in living organisms and cause extremely high toxicity [1]. As one of the most poisonous metal ions ever discovered, Hg(II) has been recognized to cause both acute and chronic toxicity to the central nervous system, kidneys, lung tissues, and reproductive system [2]. It may enter the body through gastrointestinal absorption, skin contact, or pulmonary inhalation. After entering the human body, mercury circulates in the blood and is stored in the liver, kidneys, brain, spleen, and bone, thus leading to several health problems such as paralysis, serious intestinal and urinary complexations, dysfunction of the central nervous system, and, in more severe cases of intoxication, death, widely known as the Minamata Bay accident, Japan, in 1956 [3]. Several sources including chloralkali wastewater, oil refineries, power generation plants, paper and pulp manufacturing, rubber processing, fertilizers industries, and similar industries have all contributed greatly to mercury emission into the environment [4, 5]. It has been included in the list of priority pollutants by the US EPA with a mandatory discharge limit of 10 μg Hg L−1 for wastewater and a maximum accepted concentration of 1 μg Hg L−1 in drinking water [6, 7]. Effectively removing mercury from water and wastewater is therefore very important and highly desirable.

Among the traditional techniques of removing the heavy metal ions, adsorption method has been widely studied because it is easy to operate and can be cost-effective [8–10]. To date, many adsorbents of Hg(II) have been examined. New developed adsorbents, including activated carbon [1, 11, 12], silica [13, 14], marine macroalga [15], goethite [16], bentonite [17], hydroxylapatite [18], multiwalled carbon nanotubes (MWCNTs) [19, 20], and poly(m-phenylenediamine) microparticles [21], were demonstrated to be effective for Hg(II) removal. However, most of these adsorbents suffer from low adsorption capacities and, therefore, low removal efficiencies of Hg(II). Hence, researchers have been seeking new efficient adsorbents. The polymer adsorbents with polyfunctional groups and small particle size or large surface area are more attractive due to their high capacity and fast rate of adsorption [22, 23].

The adsorbents in nanoscale usually called nanoadsorbents are in particular attractive because of their unique properties including high surface area, large pore volume, high reactivity, and good chemical species selectivity during adsorption process [24, 25]. Nanoadsorbents are the most promising materials for wastewater treatment and drinking water purification [26–28]. Nanoporous carbon (such as active carbon and multiwalled carbon nanotubes) and oxide minerals (such as iron oxide, aluminium oxide, and titanium oxide) represent two kinds of traditional nanoadsorbents. The surface area of some of these nanoadsorbents could be as high as several hundred to thousand square meters per gram, allowing sufficient contact between adsorbent particles and metal ions. However, limited functional groups present in adsorbents produce weak guest-host interactions thus leading to poor adsorption capacities [29–31]. Some surface modifications or functionalizations are needed in order to improve adsorption performance, as exampled by cysteine functionalized multiwalled carbon nanotubes (Cys-MWCNTs) [19], arginine modified TiO2 nanoparticles [32], thiol-functionalized Zn-doped biomagnetite particles [33], and citrate-coated gold nanoparticles [34]. Although these modifications are expensive and usually produce additional pollution because of the tedious steps involved [35], the modified nanoadsorbents indeed demonstrate strong adsorbability toward several heavy metal ions, but their maximum adsorption capacity, adsorption rate, and stability still need to be improved.

Nanoadsorbents made from polymer nanomaterials are more attractive because they possess not only polyfunctional groups but also high surface area. The previous report showed that poly(1-amino-5-chloroanthraquinone) (PACA) nanofibrils with high surface area could be simply synthesized (Scheme 1) [36, 37]. Nanoscale PACAs possess rigid polymer chains, extended π-conjugated structures, high charge density, and a large number of electron-rich groups including –NH–, –N=, –NH2, –C=O, and –Cl. The rigid polymer chains endow these polymers with excellent solvent resistance while electron-rich polyfunctional groups and mobile π-electrons potentially offer a strong binding affinity of polymer molecules toward metal cations. Thus, the polyfunctional nanopolymers are expected to be excellent candidates for removal of mercury ions from solutions.

To the best of our knowledge, applying aminoanthraquinone-based polymer nanomaterials for removal of mercury ions has not been reported. Herein, we report that superb adsorption capacities for Hg(II) were achieved by using PACA nanofibrils as nanoadsorbents. Adsorption mechanism studies indicated that the presence of polyfunctional groups in the polymers, high surface area, and high porosity corresponded to the superb adsorption capacity, improved adsorption kinetics, and excellent metal ion selectivity, as concluded by inductively coupled plasma atomic emission spectrometer (ICP-AES), Fourier transform-infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET) analyses. Finally, the practicability of PACA nanosorbent for the removal of Hg(II) ions from chloralkali wastewater was investigated.

2. Materials and Methods

2.1. Materials

ACA monomer, NaClO oxidant, Hg(NO3)2·H2O, thiourea, HNO3, ethylenediaminetetraacetic acid (EDTA) disodium salt, and other chemicals were procured from Chemical Reagent Corp. in China. All reagents and solvents were of analytical reagent grade and used as received. The PACA nanofibrils were obtained according to our previous reported method [37]. The nanofibril bundles had the diameters of 20–70 nm, lengths of 0.2–3 μm, apparent density of 0.375 g cm−3, bulk density of 0.649 g cm−3, and specific surface area of 35 m2 g−1.

2.2. Methods

2.2.1. Batch Adsorption Experiments

Batch adsorption experiments were conducted in 100 mL conical flasks containing 25 mL Hg(II) solution at an initial concentration of 10.0 mmol L−1, to which 40 mg polymer nanofibrils were added. The mixture was then magnetostirred at 300 rpm for 24 h in a water bath at 25°C. After adsorption, the metal loaded polymer was separated by centrifugation and the concentration of Hg(II) in the supernatant was analyzed using inductively coupled plasma atomic emission spectrometer (ICP-AES). The adsorbed amount of metal ions on the nanofibrils was calculated according to the following equations:where (mmol g−1) is the adsorption capacity, (%) is the adsorptivity, (mol L−1) and (mol L−1) are the mean ion concentrations before and after adsorption, respectively, (mL) is the initial volume of the metal ion solution, and (g) is the weight of the nanofibrils added.

2.2.2. Adsorption Kinetics

A volume of 50 mL aqueous solution containing Hg(II) at a concentration of 0.50 mmol L−1 was transferred to a glass-stoppered conical flask. After 40 mg of PACA was added, the solution was magnetostirred (300 rpm) at room temperature (25°C). After adsorption for 0–240 min, the metal loaded polymer was separated by centrifugation and the concentration of Hg(II) in the supernatant was analyzed by ICP-AES. By altering adsorption time, the procedure was repeated several times to obtain the amounts of metal adsorbed at certain intervals.

The initial sorption rates and rate constants were calculated and the sorption kinetics were evaluated using pseudo-second-order kinetics based on the equationwhere (g mmol−1 min−1) is the rate constant of adsorption, (mmol g−1) is the equilibrium adsorption capacity, and (mmol g−1) is the amounts of metal adsorbed at time .

2.2.3. Adsorption Behaviors

The adsorption behaviors of Hg(II) ion were investigated in aqueous solutions at pH of 1–7. 1.0 mol L−1 NaOH and 1.0 mol L−1 HCl solutions were used to adjust the solution pH. The equilibrium isotherms for Hg(II) ion were established in aqueous solutions at pH of 6.0 and 25°C.

2.2.4. Desorption Behaviors

Desorption of metal ions was performed by mixing PACA-metal complexes and HNO3, thiourea, or EDTA disodium salt eluent solutions of different concentrations, and the mixture was vigorously magnetostirred for 24 h at 25°C. The PACA nanoadsorbent was separated by centrifugation and the final metal ion concentrations in the supernatant were analyzed as described in Section 2.2.1. Desorption ratio (, %) was calculated from the equationwhere is the desorption ratio; (mol L−1) is the concentration of the metal ion in the desorption solution; (mL) is the volume of the desorption solution; and , , and are the same as defined in Section 2.2.1.

2.2.5. Removal of Mercury from Real Wastewater

In order to survey the capability of PACA nanoadsorbents for removal of Hg(II) from real wastewater, a batch experiment was conducted on chloralkali wastewater. The wastewater sample was taken from effluent sewage of wastewater treatment plant of Yunnan Salt & Salt Chemical Co., Ltd. After adjusting the pH to 6.0, 25 mL of sample was placed in a 100 mL conical flask and 40 mg of PACA nanoadsorbent was added and magnetostirred for 2 h. After that, the metal loaded polymer was separated by centrifugation and the concentration of Hg(II) in the supernatant was analyzed by ICP-AES.

2.2.6. Characterization and Measurements

PACA nanofibrils upon adsorption were characterized by FT-IR, XPS, TEM, and BET analyses. The FT-IR spectra of PACA as KBr pellets were recorded on a Bruker TENSOR 27 Fourier transform-infrared spectrometer (Germany) with resolution of 1 cm−1 by a transmittance method. The XPS spectra were taken by a PHI5000 Versaprobe-II multifunctional scanning and imaging photoelectron spectrometer (Japan) equipped with an Al Kα X-ray source. The size and morphology of PACA nanofibrils bundles were analyzed by a Tecnai G2 TF30 S-Twin field emission transmission electron microscope (Netherlands). The atomic emission spectra of metal ions in actual water samples were recorded on a PS1000 inductively coupled plasma atomic emission spectrometer made by Leeman Labs, Inc. (USA). The anions in actual water were determined by using DIONEX ICS-3000 ion chromatography (USA). The nitrogen adsorption/desorption measurements at 77.4 K were performed after degassing the PACA powders under high vacuum at 100°C for at least 20 h using a DZF-6020 vacuum drying oven made by Shanghai Boxun Industry & Commerce Co., Ltd. (China). The specific surface areas were calculated by applying the Brunauer-Emmett-Teller (BET) model to the adsorption or desorption branches of the isotherms (N2 at 77.4 K). The apparent (bulk) density of the PACA nanofibrils was determined by the ratio of the mass to a given volume of 2.0 cm3. The fine nanofibrils were put into a plastic tube with a scale and stacked loosely and tightly for the determination of apparent and bulk densities, respectively.

3. Results and Discussion

3.1. Effect of Adsorbent Dose

The effect of variation of the adsorbent dose on the removal of Hg(II) by PACA nanofibrils is shown in Figure 1. Adsorbent dose of PACA was varied from 0.4 to 10 g L−1 and equilibrated for 24 h at an initial Hg(II) concentration of 10 mmol L−1. It is apparent that the equilibrium concentration in solution phase decreases with increasing adsorbent dose for a given initial Hg(II) concentration. In other words, increasing the adsorbent dose significantly increased the Hg(II) ion adsorptivity. Nearly complete removal (99.56%) of Hg(II) ion from aqueous solution was attained at an adsorbent dose of 8.0 g L−1. For adsorbent dose higher than this value, the adsorptivity increased less (Figure 1). Increases in the percentage of the mercury removal with increasing adsorbent dose could be attributed to increases in the adsorbent surface areas, augmenting the number of fresh adsorbent sites available for adsorption, as already reported in several papers [38, 39]. When adsorbent dose is small, the binding sites on the adsorbent surface are small; thus the adsorptivity is low. Increasing in adsorbent dose leads to increase in active sites of metal binding which means more metal ions are adsorbed. Hence, the adsorptivity increases till saturation. Any further addition of the adsorbent beyond 8.0 g L−1 did not cause any apparent change in the adsorptivity. However, by increasing the adsorbent dose, the adsorption capacity firstly decreased slightly and then decreased considerably. The decrease in adsorption capacity can be attributed to the fact that some of the adsorption sites remain unsaturated during the adsorption process [39]. On the other hand, this may be partly due to the fact that the adsorbent gets aggregated and provides less effective surface area for metal binding [40]. In order to obtain complete saturation of the adsorbent surface with Hg(II) ion, high adsorbent dose was avoided, and all the adsorption studies were conducted at an optimal adsorbent concentration of 1.6 g L−1.

Figure 1: Effect of adsorption dose on the adsorption capacity () and adsorptivity () of PACA nanofibrils for Hg(II) ion with an initial concentration of 10.0 mmol L−1 (pH: 5.0) at 25°C.

3.2. Effect of Solution pH

The pH of the aqueous solution is an important parameter that controls the adsorption of Hg(II) ion at PACA nanofibrils-water interfaces. Note that neglectable pH variations were observed after adding PACA nanofibrils to a water with neutral pH, signifying that the nanoadsorbent itself hardly affects the acidity of the Hg(II) solutions. The effect of pH value of Hg(II) solution on the adsorption of Hg(II) ion using PACA nanofibrils as adsorbents was conducted at 25°C, as shown in Figure 2. With increasing solution pH from 1.0 to 5.0, the adsorption capacity for Hg(II) increased considerably from 0.39 to 3.05 mmol g−1. However, with further increase in pH values from 5.0 to 7.0, the adsorption capacity had neglectable changes. This could probably be explained by competition adsorption between metal and hydrogen ions with the binding sites on the surface of PACA nanofibrils [41–43]. At lower pH, binding sites especially amine/amino and carbonyl groups would be more readily protonated, resulting in weak affinity with Hg(II) ion. At higher pH, binding sites became more naked and active and Hg(II) ions were easier to be adsorbed. Notably, adsorption experiments were not conducted when the pH value was higher than 7.0, because the metal ions could precipitate or form the hydroxyl complexes [20, 44]. Given the above description and results from Figure 2, initial pH 6.0 was selected as the optimum value for Hg(II) adsorption.

Figure 2: Effect of pH on the adsorption of Hg(II) ion with an initial concentration of 10.0 mmol L−1 using 1.6 g L−1 dose of PACA nanofibrils at 25°C.

3.3. Adsorption Kinetics

The effect of contact time on the adsorption of PACA nanofibrils for Hg(II) ion was shown in Figure 3. It is apparent that the adsorption of Hg(II) is a time-dependent process. The adsorption occurred very rapidly within the initial 20 min, in which 83% adsorptivity of Hg(II) ion was achieved. Such rapid adsorption is not only because of a great number of various functional groups including –NH–, –N=, –NH2, –C=O, and –Cl on the PACA nanofibrils but also because of their high porosity which is confirmed by low density (apparent density: 0.375 g cm−3; bulk density: 0.649 g cm−3) and high specific area (35 m2 g−1) [22]. Also, this could be reasonably attributed to the absence of internal diffusion resistance [45]. At the same time, the adsorption of Hg(II) fitted well the pseudo-second-order kinetic equation (inserted curve in Figure 3); the rate constant () was calculated to be 2.277.

3.4. Adsorption Mechanism

The surface area of PACA nanoadsorbents decreased from 35 to 31 m2 g−1 after adsorption of Hg(II). The decrease in surface area initially implied that the metal adsorptions really happened. TEM observations showed that the PACA nanofibrils seemed to be slightly aggregated after Hg(II) adsorbing (Figure 4). Even so, the aggregated microparticles of PACA could be turned into nanoparticles (related TEM photograph not shown) by intense ultrasonic dispersion after metal recycling, indicating that the PACA nanofibrils could easily be regenerated.

The adsorption mechanism of PACA nanofibrils for Hg(II) was further studied by FT-IR and XPS spectral analyses (Figures 5–7). The FT-IR wavenumbers corresponding to different functional groups were summarized in Table S1 of the Supplementary Information (SI) available online at http://dx.doi.org/10.1155/2016/7245829. After adsorption of Hg(II) ions, the FT-IR stretching peaks at 3455, 1642, and 737 cm−1 due to –NH–, –C=O, and C–H groups shifted to lower energy by approximately 9, 8, and 2–5 cm−1, respectively, and the peak at 1259 cm−1 due to C–CO–C group became undetectable, indicating that some binding interaction formed between polymers and metal ions [46].

Figure 5: FT-IR spectra of PACA polymer and PACA salt after adsorption of Hg(II) at an initial Hg(II) concentration of 10 mmol L−1 with adsorbent dosage of 1.6 g L−1 at 25°C.

Figure 6: XPS wide scan spectra of PACA and Hg(II)-loaded PACA.

Figure 7: High-resolution XPS spectra of (a) , (b) , (c) , and (d) .

It is known that the main adsorption sites for Hg(II) ion are the nitrogen and oxygen atoms present in the polymer chains because they have several lone pairs of electrons that can efficiently bind a metal ion to form a metal complex, as shown in Scheme 1. The –NH– (or –N=/–NH2), –C=O, and C–CO–C groups were most likely to act as the adsorption sites for Hg(II). The reason is that soft acid of Hg(II) ion offers high affinity toward soft bases of –C=O and C–CO–C groups and middle-soft bases of –NH–, –N=, and –NH2 groups. Also, aromatic C–H bonds would be another adsorption site via formation of charge transfer between π-electron-rich anthraquinone and electron-poor Hg(II) ion.

XPS technique was applied to identify the interaction between Hg(II) ions and PACA nanoadsorbents (Figure 6). Several strong peaks with binding energies (BEs) of 200.3, 284.9, 399.4, and 531.3 eV corresponded to , , , and orbitals, respectively, which existed in pristine PACA nanofibrils. Upon adsorption of Hg(II) ion, a new peak with BE of 101.7 eV () appeared. The results further indicated that Hg(II) ion was adsorbed onto PACA.

High-resolution XPS spectra of , , , and core levels are shown in Figure 7. The detailed BEs were summarized in Table S2. It is known that when the chemical environment of atoms in a blend is perturbed by specific interaction, their binding energy will be varied, or a peak with a higher binding energy will be produced in the XPS spectrum [47]. The XPS spectra of pristine PACA displayed three peaks at BEs of 531.3, 532.5, and 536.2 eV (Figure 7(a)), attributed to –O–, –C=O, and quinone moieties [48], respectively. After Hg(II) adsorption, the BEs of for –C=O and quinone moieties decreased by 0.2 and 0.5 eV, respectively. These shifts may be due to a decrease in the electric density around oxygen atoms resulting from the electron draining to Hg(II) [49], reflecting the intense interaction between oxygen atoms and Hg(II) ions. These results along with FT-IR spectral analyses signified that Hg(II)–O coordination was formed due to the electron-donating ability of the oxygen atoms (Scheme 1, dash arrows).

The high-resolution XPS spectra of PACA upon adsorption of Hg(II) are shown in Figure 7(b). The pristine PACA exhibited a single peak located at 399.4 eV due to amine nitrogen (–NH–). After adsorption of Hg(II), however, a new peak with a higher BE of 402.1 eV appeared. Similar peaks were also found in previously reported adsorbents [48]. The reason is that some nitrogen atoms existed in a more oxidized state on the surface of PACA nanofibrils after interacting with Hg(II), forming Hg(II)⋯NH– complex (Scheme 1, dash arrows). The nitrogen atoms would then donate their lone pairs of electrons to the coordination bonds between nitrogen and Hg(II). Electron cloud density in nitrogen atoms was finally reduced, producing an additional peak with a higher BE [50]. The XPS spectra provide evidence of Hg(II) ions binding to nitrogen atoms.

The high-resolution XPS spectra of PACA upon adsorption of Hg(II) are shown in Figure 7(c). Pristine PACA showed four peaks at 284.9, 286.3, 287.6, and 291.8 eV, attributed to C–C, C–O, C=O, and large π bonds [51], respectively. The results indicate that the large π bonds also played a key role in the adsorption of Hg(II) ions (Scheme 1, dash lines). Additionally, the high-resolution spectra (Figure S1, SI) exhibited two peaks at 101.7 and 105.4 eV assigned to Hg 4f7/2 and Hg 4f5/2, respectively. The presence of Hg 4f7/2 peak at 101.7 eV indicated that Hg2+ formed a complex with –NH–, –N=, –NH2, –C=O, and large π bonds [52].

The high-resolution XPS spectra of PACA, however, exhibited neglectable changes after adsorption of Hg(II) (Figure 7(d)). Previously theoretical chemistry calculations based on density functional theory (DFT) indicated that the chlorine atoms in PACA molecules were slightly positively charged [37], so they differ from traditional electron-donating groups such as nitrogen, sulfur, and oxygen atoms. The “positive chlorine” would partly repulse the positive Hg(II) ion.

Based on the FT-IR, XPS, and DFT analyses, we conclude that chlorine atoms did not contribute to the chemical adsorption of metal ions. The adsorption mechanism could be that negatively charged functional groups including –NH–, –N=, –NH2, –C=O, and large π bonds in PACA nanofibrils coordinated with Hg(II) ions to form a stable complex (Scheme 1), leading to the superb adsorption capacity. Additionally, pH changes in metal ion solutions before and after the addition of PACA nanoadsorbents were monitored to understand the adsorption mechanism better [53]. After adsorption, the pH values of Hg(II) solutions showed unapparent changes. Accordingly, the ion exchange mechanism was not included in the adsorption of Hg(II) ion.

3.5. Adsorption Isotherms

In order to reveal the adsorption behavior and to estimate the adsorption capacity, adsorption isotherms have been studied. Since Freundlich and Langmuir isotherms were most commonly used to describe the adsorption isotherms [54], the adsorption processes of Hg(II) on PACA nanofibrils were tested with Freundlich (see (4)) and Langmuir (see (5)) isotherm models:where (mmol g−1) is the adsorption capacity based on the dry weight of nanoadsorbent, (mmol L−1) is the equilibrium concentration in solution, [(mmol g−1)(L mmol−1)1/n] is the binding energy constant reflecting the affinity of the PACA nanofibrils for metal ions, is the Freundlich constant, (mmol g−1) is the maximum adsorption capacity, and (L mmol−1) is the Langmuir adsorption equilibrium constant.

The adsorption isotherm was shown in Figure 8(a). The initial concentration of Hg(II) was in the range of 1.0–20 mmol L−1. As shown, with increasing metal ion concentration, the adsorption capacity increased correspondingly. This is probably due to a relatively higher driving force for mass transfer [55]. As shown in Figure 8(b), the plot of versus yielded a straight line with a coefficient () of 0.999, but that of versus yielded a curve with a coefficient () of only 0.944, revealing that the adsorption obeyed Langmuir isotherm rather than Freundlich one. From the slope and intercept, the values of and were calculated to be 3.846 mmol g−1 and 0.792 L mmol−1, respectively. The maximum adsorption capacity of PACA nanofibrils for Hg(II) is significantly higher than most of those reported adsorbents including polymers, inorganics, and modified inorganics, as shown in Table S3 of the SI. Note that the maximum adsorption capacities usually depend on the following two characteristics: large surface area and polyfunctional groups. Large surface area ensures the sufficient contact of the solid adsorbent with metal ions in aqueous solutions, while polyfunctional groups provide a large number of active sites for the adsorption reaction. An efficient adsorbent should optimally combine these two characteristics.

Figure 8: (a) Adsorption isotherm of Hg(II) on the PACA nanofibrils at an initial metal ion concentration from 1.0 to 20 mmol L−1, adsorbent dose of 1.6 g L−1, pH of 6.0, and temperature of 25°C for 24 h. (b) Linearized Freundlich and Langmuir isotherms for adsorption of Hg(II).

3.6. Reuse of PACA Nanofibrils

To investigate the feasibility of reusing the nanofibrils, desorption experiments were conducted. The desorption efficiency of the PACA nanofibrils was evaluated by several eluents including HNO3, thiourea, EDTA, and their combined aqueous solutions. When 0.50 mol L−1 HNO3, 3.0% thiourea, 1.0% thiourea in 0.50 mol L−1 HNO3, 3.0% thiourea in 0.50 mol L−1 HNO3, 3.0% EDTA, 1.0% EDTA in 0.50 mol L−1 HNO3, and 3.0% EDTA in 0.50 mol L−1 HNO3 were used, the desorption ratios were found to be 42.9, 59.3, 80.5, 88.0, 74.2, 90.6, and 99.9%, respectively. We found that 3.0% EDTA in 0.50 mol L−1 HNO3 was the best eluent and then used it to regenerate the nanofibrils. This is due to the fact that EDTA is a strong six-tridentate ligand containing two imino and four carboxyl groups, and the distance between these groups is enough long, which facilitates EDTA to form highly stable chelates with common transition metals. For instance, the stability constants for the complex ions of [Hg(EDTA)]2− and [Pb(EDTA)]2− are as high as 6.3 × 1021 and 1.0 × 1018, respectively [56]. On the other hand, the highly acidic environment is favorable for the desorption of heavy metal ions from the PACA nanofibrils, without significantly reducing the extremely strong complexing ability of EDTA. The adsorbent was reused in five successive adsorption-desorption cycles as can be seen in Figure 9, indicating a slight loss (1.8%) in the desorption capacity (metal recovery) for Hg(II) compared to the initial cycle (98.1–99.6% versus 99.9%), revealing a good regeneration capacity of the adsorbent. Besides, the Hg(II) adsorption capacity decreased from 2.08 mmol g−1 in initial cycle to 1.97 mmol g−1 in final cycle, which revealed a slight loss (5.3%) of adsorption capability. The data suggested that PACA nanofibrils were suitable for repeated use for far more than five times. These results suggest that PACA adsorbents were highly recyclable.

In order to evaluate the practicability of PACA nanosorbent for purifying actual wastewater, a batch experiment was conducted on chloralkali wastewater. The characteristics of chloralkali wastewater are summarized in Table S4 of the SI. As the results show, chloralkali wastewater contains a high concentration of different ions, which may interfere with adsorption of Hg(II) ion on the adsorbents. Thus, the selectivity of the adsorbent is especially important. When an adsorbent dose of 1.6 g L−1 was applied, PACA nanoadsorbent exhibited a very high capability for adsorption of mercury ions (98.4%) from chloralkali wastewater. After only one-off adsorption, the Hg(II) concentration reduced from 97.3 to 1.56 μg L−1 and thus the quality of the water is greatly enhanced, which satisfactorily meets the discharge standard of pollutants for municipal wastewater treatment plant proposed by the US EPA.

4. Conclusions

Porous poly(1-amino-5-chloroanthraquinone) (PACA) nanofibrils synthesized by interfacial polymerization demonstrated high efficiency to remove Hg(II) ion from contaminated water. Adsorption mechanism investigation indicated that high surface area and strong affinity of –NH–, –N=, –NH2, –C=O, and large π bonds of PACA nanofibrils mainly determined the metal ion removal efficiency. Adsorption kinetics studies showed that the adsorption of Hg(II) achieved equilibrium within two hours and followed the pseudo-second-order rate due to the strong chemical adsorption process; the equilibrium isotherms obeyed the Langmuir adsorption. PACA nanofibrils have been prepared by one-pot synthesis method in the present work, and relatively cheap ACA monomers, sodium hypochlorite, nitrobenzene, and alcohol were used as raw materials, respectively, while nitrobenzene was recycled for repeated use. PACA nanofibrils were highly recyclable and capable of efficiently removing Hg(II) ion after being used five times. The reagents (including nitric acid and EDTA) used in the recovery of PACA are also cheap. It is concluded that the PACA nanofibrils offer low cost and sustainable adsorbents for water purification.

Competing Interests

The authors declare that there are no competing interests regarding the publication of this paper.

Acknowledgments

This research is funded by the National Natural Science Foundation of China (Grants 51363012, 21262020, and 51203090), the Applied Basic Research Foundation of Yunnan Province (Grant KKSY201232041), and the Analysis and Testing Foundation of Kunming University of Science and Technology (Grants 2014ZX01, 2014ZX03, 2014ZX05, and 20151022).